The type of tests you choose to run to test for significance will depend on the type of variables you are using, continuous or categorical. Here's a video about variable types: http://www.youtube.com/watch?v=CNW9txOMYAg.

Specifying missing values in SPSS: http://www.youtube.com/watch?v=wfwUC0XSnUA This is particularly relevant for the students using existing data. If the data's principal investigator(s) didn't already designate values as discrete missing values, you'll need to do this.